Elastic synthetic resins made from polyisocyanates, polyhydroxyl compounds and, optionally, chain-lengthening agents are already shown. They have been described in detail in Kunststoff-Handbuch, Volume VII, Vieweg-Hochtlen, Carl-Hanser-Verlag. Many of these polyurethane elastomers can be converted into their final shape by a subsequent forming process, for example, shaped products can be produced by injection molding or by extrusion. Other methods of manufacturing shaped products include casting the reactants in molds or pressing polyurethane elastomers which can be rolled. For the continuous manufacture of panels, foils, tubes, cable sheaths and profiles, however, the main process is extrusion of thermoplastic polyurethane elastomers. These thermoplastic polyurethane elastomers are obtained by reacting polyesters or polyether diols and glycols, such as butane-1,4-diol, with approximately stoichiometric quantities of diisocyanates. This reaction must be carried out under accurately controlled reaction conditions. Apparatus for carrying out the reaction has been described, for example, in German Auslegeschrift No. 1,147,772 according to which the reactants, after they have been vigorously mixed, are introduced in accurately measured quantities into a preheated liquid bath in which they are reacted and after the reaction the products are purified and granulated and only after these stages of the process can the products be extruded to produce the finished moldings.
According to another process which is carried out in practice the reactants, which consist of polyester, butane diol and polyisocyanate, are fed into a mixing head from dosing pumps. From this mixing head, the reacting components are delivered onto a conveyor belt of polytetrafluoroethylene on which they are passed through an oven in which they are heated to temperatures of 100.degree.C. After several hours, the material is cut up into strips and then tempered for 24 hours at 100.degree.C under nitrogen. Only after this time can the products be worked up in injection molding or extrusion apparatus.
According to German Auslegeschrift No. 1,106,969, thermoplastic polyurethane elastomers are obtained by stirring stoichiometric quantities of starting components consisting of polyester, butane-1,4-diol and isocyanate for one minute, reacting them together and then pouring them into large containers which are then screwed down and heated to 140.degree.C in an oven for 31/2 hours. After cooling, the product is granulated and then worked up in extruders.
According to German Auslegeschrift No. 1,165,852, polyurethane elastomers which are capable of being thermoplastically formed may also be produced by mixing the reactants at temperatures of between 60.degree. and 135.degree.C and then solidifying the reaction mixture by pouring it on to heated metal sheets which have been coated with mold parting agents. The reacting substance is distributed over the sheet in good time so that it can subsequently be more easily detached from it. The resulting thermoplastic polyurethane compounds which are stable in storage must be reduced in size by cutting, shredding and grinding them and must finally be shaped by injection molding, extrusion or pressure molding.
The conventional processes of this kind are therefore based on working up polyurethane granulates which must be produced by complicated processes.
All attempts to obtain technically high-quality polyurethane elastomers directly from the starting components by simultaneously reacting and extruding them in screw extruders, i.e. without first producing a granulate, have so far failed. Thus for example, in U.S. Pat. No. 3,642,964 there is described a continuous one-step process for the production of thermoplastic polyurethanes in which
a. a diisocyanate, a polymeric diol, a difunctional chain-lengthening agent and a catalyst are mixed in a first zone; PA1 b. the reaction mixture is then passed through a second zone in which it is mixed under the action of high shearing forces; and PA1 c. the reaction mixture is finally continuously transferred to a forming zone in which it is formed by extrusion.
The particular feature of this process is that the temperature of the reaction mixture is adjusted so that as the reaction mixture passes through the various zones, its viscosity remains practically constant in the region of about 1000 to 10,000 Poises in all the zones.
It has been found that homogeneous products free from lumps can not be obtained by the process according to U.S. Pat. No. 3,642,964. This is in particular true if one tries to produce high-molecular weight polyureas on a technical scale by a solvent-free reaction from polyisocyanates and diamines and/or polyamines.
It has even not been possible yet to obtain undiluted high-molecular weight, elastomeric polyurethane ureas from liquid polyisocyanates and aliphatic, cycloaliphatic and/or araliphatic polyamines by the usual processes because the reaction between the aliphatic amino groups and isocyanate groups is so vigorous that the reactants cannot be homogeneously mixed before the reaction product solidifies. These polyureas have therefore in the past always been prepared as very dilute solutions. This means that either a large quantity of solvent must be transported with the reaction product from its point of production to the point where it is processed or production of the polyurea must be followed by an additional step of evaporation which involves considerable expenditure.